1. Field of the Invention
The present invention relates to methods and apparatus employing membrane filtration in biodegradation processes (called “membrane bioreactors”) for treatment of wastewater containing organic contaminates.
2. Description of Related Art
Chemical compounds containing carbon are designated as organic compounds and may include compounds derived from living organisms. Biodegradable materials are those capable of being decomposed by biological means as by bacterial action. Biodegradable materials are those that serve as food for bacteria. The bacteria are naturally occurring. Therefore, processes employing biodegradation are seen to be “nature's way” and in many cases preferred over other methods of removing organic compounds that contaminate wastewater derived from industrial and other processes.
Bacteria are active only at the limited outer surface of the contaminants to be consumed as food. The bacteria produce enzymes to disperse the contaminants and increase the amount of surface, and the amount of food, available to them. A different enzyme may be required to disperse each contaminant present.
When food is available and the right supporting conditions are present, bacteria can reproduce in large quantities in very short periods of time. Catabolism is the process by which bacteria change, or decompose, the contaminants into simpler compounds, also known as destructive metabolism. It is a chemical oxidation-reduction process for organic carbon removal that is either aerobic (those occurring in the presents of dissolved oxygen) or anaerobic (those occurring in the complete absence of oxygen). The bacteria and the man made systems in which the biodegradation processes occur are also designated as either aerobic or anaerobic. Both dissolved and solid organic wastes are decomposed to gases. The gases are carbon dioxide if the process is aerobic or carbon dioxide and methane if anaerobic. The aerobic process is much faster at the destruction of most organic contaminants and is the primary emphasis of the present invention.
The organic matter is normally a very small percentage of the contaminated wastewater and is dispersed throughout the entire content of the wastewater in which it is contained. The systems in which the biodegradation processes occur are typically very large to provide the retention time required for the bacteria to reach and consume the small percentage of organic waste. It is extremely difficult if not impossible to supply the dissolved oxygen needed to promote aerobic bacterial activity in highly diluted system.
The processes are enhanced when the organic matter can be concentrated into a smaller quantity of wastewater. A filter capable of concentrating the organic matter to an optimum level by removing a large part of the water would accelerate the biodegradation process and require a much smaller system. The suspended organic solids, dissolved organic matter, and the bacteria are called “biomass.” The container in which the biodegradation occurs is a biological reactor called a “bioreactor.” Suspended solids and dissolved minerals that precipitate from the incoming wastewater also adds to the suspended solids accumulated in the bioreactor. The filtration system selected for application in biodegradation processes is a membrane assembly called an “ultrafilter.” The system in which the membrane is applied to the biodegradation process is called a “membrane bioreactor” (MBR). Supplying the oxygen required to promote bacterial activity even in small biological process systems is a major problem experienced by the entire biodegradation process industry on a worldwide basis.
Generally, membranes are used in reverse osmosis, nanofilters, ultrafilters, and microfilters. Reverse osmosis and nanofilters membranes are able to separate dissolved ions from water and are referred to as semi-permeable membranes. Ultrafilters and microfilters have porous membranes and accomplish separation mechanically. The ultrafilter can remove suspended solids and dissolved substances with a cutoff point by molecular weight depending on the size of the pores. Membranes are synthetically produced from materials selected to have specific properties. The membranes are manufactured in the form of tubes, hollow fibers, and sheets.
In all membranes water is kept flowing along the surface of the membrane in a sweeping or washing motion called “cross flow” to prevent excessive concentration of solids on its surface. The cross flow results in shearing the solids from the membrane surface and prevents fouling.
The membranes are assembled in a configuration called a module or an element. The module, or element, is the smallest assembly that can be purchased, and is, therefore, referred to as the membrane. Applying a membrane coating on the inside of porous tubes produces tubular membranes. The coating becomes the membrane and the porous tube serves as the backing or reinforcement for the membrane surface. The feed wastewater enters the tube and some of the water is separated from the solids and passes through the membrane and porous tube. The water that passes through the membrane is called “permeate.” The part of the biomass including wastewater, suspended solids, and dissolved organic solids that do not pass through the membrane as permeate is called “reject.” The amount of permeate a membrane can produce per unit of membrane area is called “flux rate” and is a function of the differential pressure across the membrane and the effectiveness of the cross flow at keeping the membrane from fouling.
In applications where wastewater has high solid concentrations, the tubular membrane has the advantage of allowing high velocity cross flow inside the tubes to sweep the solids and minimize sticking to the membrane surface and reducing the capacity. The result of the deposit on the membrane surface is called “fouling.” While the tubular membrane is used in the disclosure of the present invention it is understood that it is not intended to be a limitation of the invention.
Hollow fiber membranes may be very small in diameter. The thin outside skin is the membrane and the porous layer acting as the support medium. Feed water is introduced around the outside of the hollow fiber. The water is separated from the solids as it passes through the thin outer layer, flows through the porous support layer to the inside of the hollow tube, and then out the end. The hollow fiber module contains an enormous amount of fibers that account for the large surface area of this type membrane.
Sheet membranes are made by modifying the surface of a thicker sheet to form a dense, microporous film on top and serves as the working membrane that actually rejects the solids and large molecular weight solutions as water flows through. The function of the remaining and thicker part of the sheet is to provide support. The sheet membranes are made by winding two large sheets in a spiral, called “spiral wound membranes,” with two membrane surfaces facing each other and placing a spacer between the support sides of the two sheets as they are rolled into a coil. The spiral wound membrane has not played a significant roll in the wastewater treatment industry. The sheet membranes are also made in the form of discs. The discs are assembled by stacking the discs in a module. The disc membrane has some application in wastewater treatment.
Membrane bioreactors have been used commercially for 20 years or more. The European and other foreign industries appear to be ahead of the United States in developing the technology and applying it to biodegradation processes. The state of technology commercialization is well documented in “Membrane Bioreactors for Wastewater Treatment,” Tom Stephenson, Simon Judd, Bruce Jefferson, and Keith Brindle, IWA Publishing, London, UK, ©2000 IWA Publishing and the authors.
The biological processes that allow the entire biomass to be in free suspension in the wastewater are called “suspended growth” processes. Good mixing is required in suspended growth systems to ensure bacterial contact with the entire organic content of the systems. In other biological processes fixed structures of material not consumed by bacteria are provided on which the bacteria can grow. Those biological processes are classified as “fixed film” processes. The wastewater containing the organic matter in fixed film processes has to be brought into contact with the bacteria on the support structures, therefore, also requiring some form of mixing.
Membrane bioreactor systems are also identified by the function of the membranes applied and where they are located in the systems. There are membranes used for separation of biomass, some are submerged in the biomass and used as a fixed film through which bubble-less pure oxygen is supplied to the bacteria attached to the outside of the membranes, and some are used for extracting inorganic materials that may inhibit the activity of the bacteria in degradation of certain toxic compounds in wastewater. The methods and apparatus of the present invention corrects deficiencies of the biomass separation systems, enhances energy operating efficiency, and may totally replace the need for most submerged fixed-film membrane bioreactor systems used to supply bubble-less pure oxygen to the bacteria. The extracting membrane bioreactors are applied for special purposes and can be used in conjunction with or without the other membrane bioreactor processes. Therefore, extracting membrane bioreactor processes are not addressed in the present invention.
In biomass separation bioreactor systems the membranes are used to separate and concentrate the biomass by removing wastewater. In one type separation system the membranes are located in a sidestream outside the bioreactor. The biomass is drawn from the bioreactor and pumped through the membranes where wastewater is removed as permeate and suspended organic and mineral solids, dissolved organic matter, and bacteria are retained and returned to the bioreactor more concentrated. In this configuration, the biomass is under pump pressure when flowing through the membranes and provides the differential pressure across the membrane.
In other biomass separation systems the membranes are submerged in the bioreactor. The head pressure of the wastewater on the outside of the submerged membranes provides lower but sufficient differential pressure to drive the wastewater through the membranes and concentrate the biomass in the bioreactor. In some submerged systems the head pressure is supplemented by a suction pump connected to the permeate outlet side to create a higher differential pressure across the membranes.
The solids in the biomass (sometimes called “sludge”) build up on and foul the membranes, and the systems must be periodically shutdown and chemically cleaned. The more concentrated the sludge becomes the more often the membranes have to be cleaned. A method of reducing the concentration of sludge flowing through the membrane would alleviate the clogging and greatly reduce the amount of membrane cleaning required and reduce the cost of chemicals used in the process. Increasing the operating time between cleaning operations would also reduce the cost of manpower and increase the useful life of the costly membranes.
The bioreactor is typically a tank or vessel in which biological reduction of organic matter occurs. The tank is aerated to supply oxygen to the bacteria and to help with mixing. Increasing the pressure under which the bioreactor operates also increases the amount of oxygen that can be dissolved in the water. However, the cost of the bioreactor vessel increases dramatically with increased pressure and size.
Aeration is a major problem typically experienced by the entire wastewater treatment industry. Hundreds and perhaps thousands of people are looking for a better way to provide the oxygen required by bacteria in all types of biodegradation processes including those that employ membrane bioreactors. While separation membrane bioreactors typically provide oxygen by bubbling air through the biomass, the present invention brings the bacteria into direct contact with dissolved oxygen by circulating the entire volume of biomass through liquid-gas mixers applied as dissolved gas generators. Either oxygen in air, pure oxygen, or enriched oxygen can be supplied under pressure to the dissolved gas generators by a compressor, bottled oxygen, or membrane separators that enrich the oxygen by removing nitrogen from atmospheric air. Alternately, the dissolved gas generators can also draw oxygen by suction from the atmosphere or from other low-pressure supplies. The use of dissolved oxygen for aeration alleviates the serious problem of foaming typically experienced by bubbling air through the systems.
A need exists worldwide for an improved apparatus and method of providing oxygen to bacteria and reducing sludge fouling of membranes in membrane bioreactor systems.
It will become clear to those skilled in the art having the benefit of this disclosure that the methods and apparatus in accordance with the present invention overcome, or at least greatly minimize, the deficiencies of existing membrane bioreactor apparatus and methods.